This invention relates to tactile sensors and more particularly to a compliant tactile sensor that delivers a force vector with three components.
Tactile sensors are used for robotic manipulation and to sense interactions with human hands or pen-like interfaces. We are motivated to build tactile sensors that are useful for robotic manipulation in unmodeled environments [12]. The numbers in brackets refer to the references appended hereto, the contents of all of which are incorporated herein by reference. These sensors should be capable of detecting when a robotic part (i.e. finger, hand, arm, etc.) comes in contact with any type of object at any incident angle. This feature is very important because in general a robot will not have any prior model of the object and thus have to use its limbs to come into contact and learn about the object. Moreover, the sensor should have a shape that makes it prone to contact.
Current tactile sensors are not good at detecting generalized contact. For example, consider a computer touch pad that uses force resistor sensors (FSR). These pads have high spatial resolution, low minimum detectable force (about 0.1 N) and a good force range (7 bits). These features make the sensor work very well when a human finger, a plastic pen or another object with a pointy shape comes in contact. However if one places a larger object or the same pen at a small incident angle, it is very unlikely that these contacts are detected unless the applied force is very large.
The detection is even more difficult when this pad is mounted in a low mechanical impedance robotic finger (such as the one in Obrero [12]). This is because the low mechanical impedance makes the finger deflect when it comes in contact with an object. A tactile sensor on this finger should be able to detect this contact with only a little deflection of the finger, because if the deflection is large, undesired forces are already being applied to the object. Therefore, the sensor needs to be very sensitive and able to detect forces applied from different directions.
Another factor that we consider important for the functionality of the sensor is its shape. The shape should be such that it is physically reachable from a range of directions. In other words, the shape should make the sensor prone to contact. We can observe that in the human skin, there are hairs and ridges that stick out. That is opposed to the design of a traditional tactile sensor whose shape is planar and only normal forces to its surface will be detectable.
Moreover, after the initial contact with an object, the fingers of a robot exert high forces to handle objects. Consequently, the tactile sensors also need to deal with this condition by either handling saturation or having a large operating range. Lastly, a tactile sensor should be able to conform to the object to increase the friction and facilitate manipulation.
Many attempts have been made to implement tactile sensing in robots. There are many technologies used to build sensor arrays: conductive elastomers [11], elastomer-dielectric capacitive [13], optical sensors (surface motion and frustrated internal reflection) [9], piezoelectric [7], acoustic, magnetoelastic, electromagnetic dipoles, silicon micromechanical (mems), and force sensing resistors. A complete review of these technologies can be found in [5]. The performance of these sensors has been measured according to the parameters mentioned in a survey study by Harmon [4]. Those parameters include: spatial and temporal resolution, measurement accuracy, noise rejection, hysteresis, linearity, number of wires, packing, and cost. However, it is not clear if any of these designs are useful for manipulation because little attention has been given to the data produced by these sensors [5].
Most sensors are essentially a flexible elastic skin, coupled with a method of measuring the deformations caused by the applied force. In [3], an IR emitter and receiver are placed inside cavities of an elastic skin. As the skin deforms, the cavity shape changes resulting in change of the received signal. However, the results on pressure sensitivity are not shown, and the sensor is not able to distinguish the direction from which the pressure is applied. In [9] the idea is taken one step farther. A full matrix of such sensors is developed and analyzed, light being routed in and out of the cavities using a matrix of optical fibers, taking out some of the bulk of the sensors. Again, only scalar data is obtained.
An interesting idea for a tactile sensor is described by both Lang [10] and Hristuy [2]. Here the finger is a white flexible membrane with a pattern of black dots or lines drawn on the inside. A light source and a CCD camera are placed inside the finger facing the patterns. Pressure on the membrane results in deformations that are detected by analyzing the CCD images. Significant processing power is required, and overall the sensor is quite big and expensive.
Another approach for the sensing tactile forces has been to use joint torque and force information to recover the normal forces [1] instead of using superficial sensors. Nevertheless, this approach is only able to detect resulting forces as opposed to distributions. Tactile sensors also have been developed using organic materials to print circuits [11]. This approach creates flexible transistors that can be used to develop a flexible skin with a high density of touch sensors that can easily be wrapped around a manipulator's fingers. The sensor used is a rubber sheet that changes conductivity with deformation. The transistors' role is to locally amplify the signal and connect it to the matrix of wires that routes the signals to the controller. While the idea has great potential to be developed, so far the results are modest in terms of sensitivity, as they only detect force applied in one direction and the organic technology still needs to be perfected (they are sensitive to oxidation).
A promising tactile sensor is described by Chu [13]. Essentially the skin is padded with rubber cones each placed on top of four capacitive force sensors. Having four sensors under each cone makes the detection of both the perpendicular as well as the sideways force possible. The results were very encouraging with sub-gram forces detected. However, the process requires a custom silicon wafer to be made (for the capacitive sensors) and therefore might be prohibitively expensive.
A unidirectional capacitive sensor is described by Voyles [8]. Here an electrorheological gel is placed between the inner and outer membrane padded with electrical contacts. The inner membrane is a hard material (like plastic) while the outer material is elastic (rubber). Again, pressure changes the relative position of the contacts which causes a change of the capacitance. One interesting property of this gel is the change in viscosity with the electric field applied, opening doors for the adaptive skin strength to the task. Unfortunately no quantitative data on the sensor are published.